Polyoxymethylene copolymers have excellent properties including mechanical properties, thermal properties, electric properties, sliding properties and molding properties and hence can find a broad scope of applications as structure materials and structure parts in the field of electric appliances, automobile parts, and precision machine parts. Particularly, the scope of applications of such resin materials has been broadened remarkably further in recent years. Then, by turn they are required to show better performances at lower cost as well as higher properties.
One of the serious problems from the viewpoint of the quality requirements of polyoxymethylene copolymers is that formaldehyde gas is generated during a polyoxymethylene copolymer molding process as the polyoxymethylene copolymer is thermally decomposed in the molding machine. Formaldehyde gas can adversely affect the human health, giving rise to a so-called sick house syndrome and other health problems. The ministry of Health, labor and welfare of Japan issued a guideline for the indoor formaldehyde concentration to be 0.08 ppm as a countermeasure against the sick house syndrome. Thus, final products of polyoxymethylene copolymer are required to produce formaldehyde only to a minimal level. Various processes for producing polyoxymethylene copolymers that produce formaldehyde gas only minimally have been proposed to date. They include, for example, a method of polymerizing monomers containing impurities only to a reduced level and rapidly cooling the produced polymer immediately after the polymerization to inactivate the catalyst and suppress any side reaction (Patent Document 1), a method of directly adding water to the extruder for terminal stabilization (Patent Document 2) and a method of polymerizing monomers to which sterically hindered phenol is added, subsequently inactivating the catalyst by controlling the polyoxymethylene copolymer after the polymerization to an optimum particle size, adding water and then devolatilizing the molten product under reduced pressure for terminal stabilization (Patent Document 3).
While polyoxymethylene copolymers showing a high yield of polymerization not less than 95% in particular are advantageous from the viewpoint of productivity and economy, they produce a number of thermally instable structures during the polymerization process so that the product is thermally unstable and can generate formaldehyde to a large extent in the molding machine. While it is possible to suppress the generation of formaldehyde in the molding machine by reducing the yield of polymerization, a reduction of yield of polymerization not only lowers the productivity but also gives rise to cost of collecting monomers to a great economic disadvantage. Thus, currently it is not possible to improve the thermal stability of polyoxymethylene copolymers containing a large number of thermally instable structures by means of any of the above listed thermally stabilizing methods.
Popularly known techniques for suppressing the decomposition of polyoxymethylene copolymers by means of one or more than one additives include those of adding one or more than one sterically hindered amine compounds or sterically hindered amine compounds as antioxidants and one or more than one polyamides, urea derivatives and/or hydroxides of any of alkali or alkaline-earth metals as thermal stabilizers. However, no satisfactory results can be obtained in terms of decomposition stability when the thermal stability of the polyoxymethylene copolymer itself is poor. Additionally, the use of additives to a large extent is economically disadvantageous.
Techniques of adding various compounds having reactivity with formaldehyde are known for the purpose of suppressing the generation of formaldehyde from pellets and molded products of polyoxymethylene copolymer. For example, hydrazide compounds are known to be highly reactive with formaldehyde and effective for immobilizing formaldehyde (Patent Document 4). However, the above-cited Patent Document describes only the reactivity of general hydrazide compounds with formaldehyde and does not describe about means for efficiently obtaining a polyoxymethylene copolymer with a reduced degree of formaldehyde generation.
Other known techniques of using a hydrazide compound as a formaldehyde scavenger include a method of compounding a borate of a nitrogen-containing compound such as hydrazide (Patent Document 5), a method of using 1,2,3,4-hydrazide butanetetracarboxylate that is a novel compound operating as a formaldehyde adsorbent (Patent Document 6) and a method of compounding a hydrazide compound and urea or a urea derivative to a specific ratio (Patent Document 7). However, any of these techniques are unsatisfactory for effectively suppressing the generation of formaldehyde and/or accompanied by a problem of degrading the physical properties of molded products of polyoxymethylene copolymer.
The inventors of the present invention found in the past that a polyacetal resin composition that contains a hydrazide compound and an amine-substituted triazine compound to specific respective ratios can effectively suppress the generation of formaldehyde from pellets and molded products of polyoxymethylene copolymer (Patent Document 8). However, the process using such a composition requires a large number of extrusion steps and hence there is a strong demand for a method of economically producing a polyoxymethylene copolymer that generates formaldehyde only at a low rate with a smaller number of extrusion steps.
Patent Document 1: Jpn. Pat. Appln. Laid-Open Publication No. 5-247158
Patent Document 2: Jpn. Pat. Appln. Laid-Open Publication No. 7-233230
Patent Document 3: Jpn. Pat. Appln. Laid-Open Publication No. 10-168144
Patent Document 4: Jpn. Pat. Appln. Laid-Open Publication No. 04-345648
Patent Document 5: Jpn. Pat. Appln. Laid-Open Publication No. 10-086630
Patent Document 6: Jpn. Pat. Appln. Laid-Open Publication No. 06-080619
Patent Document 7: Jpn. Pat. Appln. Laid-Open Publication No. 2002-035098
Patent Document 8: Jpn. Pat. Appln. No. 2005-267414